c# - Use of unassigned local variable: value type vs custom struct

Question

A primitive C# value type, for example int is a struct. So, why is int not initialized? There supposed to be default constructor, I think. On the other hand, a custom struct is ok.

In the following code

struct STRCT { }
class Program
{
    static void Main(string[] args)
    {
        STRCT strct;
        strct.Equals(8);
        strct.GetHashCode();
        strct.GetType();
        strct.ToString();

        int i;
        i.Equals(8);
        i.GetHashCode();
        i.GetType();
        i.ToString();
    }
}

while first 5 lines of code are ok from the C# compiler view, next 5 lines of code generates compile error:

use of unassigned local variable

Please, explain why? From my point of view both types are structs and shall have the same behavior.

Answer 1

It's a pathological extreme of the rules of Definite Assignment. Specifically:

A struct-type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.

In this case (STRCT strct), the set of instance variables is empty, and so it is true that they've all been definitely assigned.